Exposures to airborne asbestos and man-made vitreous fibers (MMVFS) increase the incidence of lung cancer, asbestosis, and mesothelioma. Fibers that deposit in the bronchial and alveolar regions, subsequently translocating to the parenchyma, are thought to be responsible for the development of these diseases. Physico-chernical properties of fibers, including length, diameter, and durability in the lung, are major factors in the etiology of these lung diseases. Because inhalation is the main route of exposure, the deposition pattern in the respiratory tract as a function of fiber dimensions is new information critical to understanding respiratory dosimetry and defining the index of exposure for health protection purposes. Controlled studies of fiber deposition in human volunteers are not available because of ethical concerns. However, total and regional depositions of inhaled fibers have been estimated from postmortem measurement, mathematical modeling, and animal toxicity studies. Increasingly, mathematical deposition models have been used to assess the dosimetry of inhaled MMVFS. However, current lung dosimetric models for fibers in the human respiratory tract are based on theoretical equations, which have not been verified with experimental data. This proposal has three objectives: (1) to develop experimental information on the deposition of fibrous aerosols as a function of fiber diameter and length in realistic human respiratory tract replicas, (2) to verify and improve the prediction of fiber dose estimate in human lungs using both empirical data as well as a computational fluid dynamic technique, and (3) to define,.a size-selective exposure index based on fiber penetration data. Because lung diseases caused by inhaled fibers occur in the bronchial, alveolar, and parachymal regions, a thoracic fraction defined as the fraction of particles penetrating the larynx and reaching the lung must be established and will be defined from experimental data obtained in this study. This research will generate essential information on the dosimetry of inhaled fibers in the human lung, data for an improved mathematical lung deposition model, and a definition of the thoracic fraction of fibers for exposure assessment. Sampling devices based on this size-selection definition can be developed in the future for improved assessment of worker exposure.